What strange creatures dwell in the rain forests, at the bottom of the ocean or even in plain sight in our cities? If we don't look, we'll never know, one group of researchers says.

A study published Jan. 24 in the journal Science suggests that discovering and recording all of Earth's biodiversity may not be as difficult as previously thought, and could be accomplished with a "realistic surge of effort," said study co-author Mark Costello, a researcher at New Zealand's University of Auckland. By spending between $500 million and $1 billion annually for the next 50 years, humans could describe most species on Earth, Costello told OurAmazingPlanet.

Costello and his two co-authors also calculated that extinction rates are not as high as many scientists previously thought. The study suggests that species are currently being discovered faster than they go extinct, contradicting a widely held tenant amongst scientists that the opposite is currently happening amid the biggest mass extinction since the dinosaurs were wiped out tens of millions of years ago. Though some scientists welcome the focus Costello and his colleagues are placing on the need to catalog Earth's species, they don't necessarily agree with their conclusions.

How many species are there?
Estimations of the number of species that live on Earth vary considerably, from as few as 2 million to as many as 100 million species. Costello's paper suggests there are between 2 million and 8 million species, at the low end of many scientists' estimates. It is difficult to tell exactly how many species there are without counting them, of course; different environments (many little-studied) have different levels of biodiversity, making it difficult to come up with a global number, and little is known about remote environments such as the deep sea, for example.

There are currently more than 1.5 million species described, but the exact number is uncertain due to overlapping descriptions of the same species, as well as the lack of digitization of many databases and collections, said Mike Novacek, the provost of science at the American Museum of Natural History in New York, who was not involved in the study. [ Earth Quiz: Do You Really Know Your Planet? ]

Although the amount of money Costello and his colleagues say will be needed to count Earth's species may seem like a lot, it pales in comparison to what we spend on sports, entertainment and space exploration. Knowing how many species are on Earth is vital to understanding life itself, Costello said.

"It's part of exploring our own planet. It is the first step in understanding ecosystems and as fundamental to biology as naming particles is to physicists, or describing elements is to chemists," he said.

Novacek said that he welcomed the paper's emphasis on recording species and conservation. "It's a cultural embarrassment that we know so little about life on this planet," he said. However, the paper's estimates of species extinction were a little low, he added.

Camilo Mora, a biologist at the University of Hawaii, went further, saying he thought the study significantly underestimated the number of extinctions occurring worldwide, making the current extinction crisis appear less worrisome than it is.

Extinction rates are also important to know because every organism serves a unique role in its ecosystem, which suffers when species are lost. Healthy ecosystems can make for cleaner water and air, as well as ensure the survival of important resources. Even people in cities and towns reap the benefit of far-flung biodiversity; for example, many modern drugs (like quinine, used to treat malaria) have originated from chemicals found in rain forest plants.

The study
The new study was a review of newly published research on extinction rates and discoveries of new species. Costello said that his team's approach was novel because it attempted to calculate global levels of biodiversity by looking at the sum of individual ecosystems the world over. Other calculations of extinction may have overstated the problem by taking local numbers and applying them globally, which Costello's team took pains not to do, he said. High levels of biodiversity in one patch of rain forest may not be paralleled in other areas of rain forest or temperate forest, for example, he said. [ 8 of the World's Most Endangered Places ]

Costello's team also suggests that there are more papers than ever describing new species, thanks to the involvement of a growing number of scientists who don't typically specialize in taxonomy, as well as amateur scientists, he said. For that reason, the task of describing the world's species may not be as insurmountable as thought, he added.

Observed rates of extinction haven't been as high as predicted by some, due in part to better conservation efforts worldwide and the survival of animals in "secondary" habitats like agricultural areas, Costello said. Species can hang on in these degraded habitats longer than expected, giving conservationists a chance to save them before they disappear, he said. Pristine habitats are nevertheless vital to protect, he added.

Controversy
But not everyone agrees with the assessments and conclusions of Costello and his co-authors.

Even the median rate of extinction suggested in Costello's paper — at 25,250 per decade — is disturbing for the planet, Novacek said, while the lower bound of the estimate (500 extinctions per decade) sounded a little low and was "optimistic," to say the least.

Mora's criticism went further: "They paint a very nice glossy picture of the reality of what's happening out there," Mora told OurAmazingPlanet. "But it doesn't represent the reality."

For example, Mora said his "mind was blown" (in a negative way) by the 500-extinctions-per-decade suggestion. Habitat loss alone leads to 25,000 extinctions per year, he said. "And that's just because of habitat loss. Now start adding all the stressors — like climate change, invasive species, pollution — and the number is likely to go a lot higher," he said.

Mora also disagreed with the paper's assertion that the number of qualified taxonomists is growing worldwide. While there may be more authors of papers describing new species, many of these consist of amateurs or nontaxonomists who do not have the necessary expertise to provide leadership in the field, he said. There are fewer full-time positions for taxonomists and many experts in their fields aren't being replaced once they retire, Mora said, a view with which Novacek agreed. [ Amazing Species Discovered in 2012 ]

Extinction crisis
All sides could agree, however, that we are in the midst of an enormous extinction crisis, the largest since the disappearance of the dinosaurs 65 million years ago, and that we must do more to record and conserve these species before they vanish. "The dinosaurs disappeared because of an asteroid, and in this case we are the asteroid," Novacek said.

All sides agreed that humans could — and should — record most species, although opinions on exactly how much effort or money it might take differed. In the short term, smaller efforts could make a big difference, Costello said.

"We estimate the backlog in undescribed species in collections could be cleared by hiring 500 new taxonomists for 10 years," he said, which would cost about $5 million per year, and help pave the way for the more expensive and time-consuming process of describing new species found in the wild.

"In the end, there's going to be some controversy and dialogue about these numbers, but I'm glad the paper is coming out and that the issue (of extinction and conservation) is being discussed, because it's so important," Novacek said.